Nickel and cobalt are generally found together in natural-occurring minerals and, because conventional ore dressing methods do not effect a separation of the two, both metals generally appear together in solutions resulting from the leaching of nickel and cobalt-containing materials, such as leached oxide ores, oxidized sulfide concentrates and the like.
In recent years, several hydrometallurgical methods have been proposed for the recovery of nickel and/or cobalt from lateritic limonitic ores. One method, in particular, resides in using aqueous sulfuric acid as the leachant at high temperature under elevated pressure. The raw ore is prepared in a finely divided state and a slurry formed at about 10 percent to 20 percent solids which is thereafter concentrated by settling and decanting in thickeners to produce an underflow having a concentration of about 30 percent to 50 percent solids. The concentrated slurry is heated in an autoclave by means of direct high pressure steam to a high temperature at which the leaching or other recovery treatment is carried out, usually above 400.degree.F (205.degree.C), e.g. about 475.degree.F (246.degree.C) at a pressure of about 525 psig in the presence of sulfuric acid to solubilize the nickel and cobalt present in the slurried ore. Following leaching in the autoclave, the leached pulp is cooled and preferably washed by countercurrent decantation and the resulting acid leach liquor then treated with a neutralizing agent [Mg(OH).sub.2, coral mud, or the like] to raise the pH to, for example, 2.5 to 2.8 for the sulfide precipitation of nickel and cobalt. The leach liquor is brought to a temperature of about 250.degree. F (122.degree.C) and the nickel and cobalt precipitated as sulfides with H.sub.2 S at pressures of up to about 150 psig, using nickel sulfide as seed material.
The sulfide precipitate is washed and thickened to about 65 percent solids and then oxidized at about 350.degree.F (177.degree.C) and a pressure of about 700 psig in an autoclave in 1 percent sulfuric acid. Ammonia is added as a neutralizing agent to the nickel-cobalt solution to raise the pH to a level (e.g. 5.3) using air as an oxidant, to precipitate any iron, aluminum or chromium carried over as an impurity during leaching. After separating the solution from the precipitate, any copper, lead or zinc present therein is removed by precipitation as a sulfide, using H.sub.2 S as the precipitant, the solution being first adjusted with acid to lower the pH to about 1.5. The sulfide precipitate is then separated from the solution and the solution then passed on to the nickel separation step.
Many methods have been proposed for the separation of nickel from cobalt contained in said aqueous solution. In one method, the solution is adjusted with ammonia to prepare it for the recovery of metallic nickel. The adjusted nickel feed solution containing about 40 to 50 grams per liter of nickel and some cobalt is reduced with hydrogen in an autoclave at about 375.degree.F (190.degree.C) and 650 psig using nickel powder as seed material, the barren liquor remaining going to cobalt recovery using known methods. However, some of the cobalt appears in the reduced nickel product.
Other methods proposed and/or commercially used for separating cobalt from nickel include cobalt sulfide precipitation from ammonia-ammonium carbonate solutions, cobalt separation from aqueous sulfuric acid solutions by means of nickelic hydroxide, and the separation of cobalt from nickel using the pentammine process described in U.S. Pat. Nos. 2,767,053 and 2,767,054, the cobalt being thereafter preferentially recovered in the metallic state by hydrogen reduction. This method, in particular, involves forming a solution containing cobaltic pentammine and nickelous ammine by adding an amount of ammonia sufficient to provide 5 mols of ammonia for each mol of cobaltic ion and 5 mols of ammonia for each mol of nickelous ion. The solution is oxidized with oxygen and then acidified with H.sub.2 SO.sub.4 to a pH of at least about 4 to produce a nickel-bearing precipitate in the form of a nickel-ammonium double salt.
While the foregoing methods provided an acceptable nickel produce, provided the end use tolerated the presence of cobalt in the nickel, these methods in and of themselves had certain limitations insofar as the production of a pure nickel product was concerned.
For example, the recovery of nickel by the hydrogen reduction of nickel-cobalt solutions was not selective enough to meet new specification requirements for high purity nickel. The sulfide and the pentammine precipitation processes are generally limited to specific nickel-cobalt bearing solutions, the sulfide precipitation method generally requiring the use of significant amounts of a sulfidizing agent (e.g. H.sub.2 S). The nickelic hydroxide procedure is quite expensive and generally requires considerable amounts of caustic soda and oxidizing agents and has a very slow filtration rate. Both the nickelic hydroxide and the sulfide precipitation processes usually result in significant loss of nickel together with the precipitated cobalt in that the cobalt-containing precipitate contains more nickel than cobalt.
The present invention provides an improved method of separating cobalt from nickel-cobalt solutions.